1. Field of the Invention
Generally, the field of the present invention is semiconductor diode lasers. More particularly, the invention relates to flared laser oscillator waveguides.
2. Background
Multimode laser diodes, also known as broad area lasers (BALs), have the property that their slow-axis beam-parameter-product (BPP) and their slow-axis brightness (power÷BPP) degrade progressively when they are driven at higher current to generate higher power. Brightness can be improved in BALs by reducing the emitter width; however, the current at which the maximum brightness occurs also happens at progressively lower current values. Hence, the maximum output power at the maximum brightness also drops. For power-scaling applications and reducing the cost-per-watt of producing diode lasers, higher brightness at higher output power per emitter is very desirable.
Semiconductor diode lasers are formed by growing multiple layers of semiconductor materials on a suitable substrate with a lattice constant that allows choice of materials to produce desired emission wavelengths. A typical semiconductor laser comprises n-type layers, p-type layers and an undoped active layer between them such that when the diode is forward-biased, electrons and holes recombine in the active region layer to produce light. The active layer (quantum well(s), quantum wire(s) or quantum dots, type-II quantum well(s)) resides in the waveguide layer which has a higher index of refraction compared to the surrounding p- and n-doped cladding layers. Light generated from the active layer is confined in the plane of the waveguide.
A conventional edge-emitting Fabry Perot broad area laser diode is arranged as a rectangular gain or index-guided semiconductor structure. Opposing end facets of the waveguide define high and partial reflectors to provide feedback for oscillation of light within the resonator. The multi-layered semiconductor laser diode structure extends the length of the laser and has a broad width for electrical injection extending to opposite side surfaces which also extend the length of the laser. The multi-layered semiconductor materials are typically arranged so that the laser operates in a single mode along the growth direction of the laser and this direction is defined as fast-axis direction. Since along the fast-axis direction the semiconductor laser operates in a single mode, the brightness of laser diode in this direction cannot be improved any further—it is so called diffraction-limited. The distance between the top and bottom surfaces of the multi-layered semiconductor laser structure thus provides the smaller dimension of the end facets, i.e., the thickness of the stripe, typically on the order of microns. On the other hand, the width of the multi-layered laser structure provides the larger dimension of the end facets, i.e., the stripe-width is typically on the order of many tens of microns to hundreds of microns. Because the stripe width is much larger than the wavelength of light, the lateral property of an optical field propagating along the optical axis of the waveguide is highly multimode along the longer stripe dimension and the corresponding axis is described as slow-axis.
Diode laser ridge waveguide structures with single-mode structural characteristics across the slow-axis have been described which may be suitable for lower powers where single-mode performance is desirable. For example, in U.S. Pat. No. 6,014,396 to Osinki et al. a flared semiconductor optoelectronic device is disclosed that has a double-flared structured. Other examples of conventional ridge waveguide structures can be found in U.S. Pat. Nos. 7,623,555 and 6,798,815. These devices have single mode beam quality in both directions but such performance comes at the expense of limited output power. However, the problem of scaling to higher powers while maintaining superior brightness continues to pose a challenge in the art of diode lasers, particularly where devices are highly multimode across the slow axis, and so a need remains for improvements associated therewith.